Firing metal with support

ABSTRACT

A solar cell can include a substrate and a semiconductor region disposed in or above the substrate. The solar cell can also include a conductive contact disposed on the semiconductor region with the conductive contact including a paste, a first metal, and a first conductive portion that includes a conductive alloy formed from the first metal at an interface of the substrate and the semiconductor region.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit coupledthereto.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present disclosure allow forincreased solar cell manufacture efficiency by providing novel processesfor fabricating solar cell structures. Some embodiments of the presentdisclosure allow for increased solar cell efficiency by providing novelsolar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a cross-sectional view of a portion of an examplesolar cell having conductive contacts formed on emitter regions formedabove a substrate, according to one embodiment.

FIG. 1B illustrates a cross-sectional view of a portion of an examplesolar cell having conductive contacts formed on emitter regions formedin a substrate, according to one embodiment.

FIGS. 2A-2D illustrate cross-sectional views of a portion of an exampleconductive contact, according to one embodiment.

FIGS. 3A-3E illustrate cross-sectional views of a portion of anotherexample conductive contact, according to one embodiment.

FIGS. 4A-4C illustrate cross-sectional views of fabricating solar cellshaving conductive contacts, according to one embodiment.

FIG. 5 is a flowchart illustrating an example method of forming aconductive contact, according to one embodiment.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. §112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” conductive portion of a solar cell does not necessarily implythat this conductive portion is the first conductive portion in asequence; instead the term “first” is used to differentiate thisconductive portion from another conductive portion (e.g., a “second”conductive portion).

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While B may be a factor that affects the determination of A, such aphrase does not foreclose the determination of A from also being basedon C. In other instances, A may be determined based solely on B.

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

Although much of the disclosure is described in terms of solar cells forease of understanding, the disclosed techniques and structures applyequally to other semiconductor structures (e.g., silicon wafersgenerally).

Solar cell conductive contacts and methods of forming solar cellconductive contacts are described herein. In the following description,numerous specific details are set forth, such as specific process flowoperations, in order to provide a thorough understanding of embodimentsof the present disclosure. It will be apparent to one skilled in the artthat embodiments of the present disclosure may be practiced withoutthese specific details. In other instances, well-known fabricationtechniques, such as lithography and patterning techniques, are notdescribed in detail in order to not unnecessarily obscure embodiments ofthe present disclosure. Furthermore, it is to be understood that thevarious embodiments shown in the figures are illustrativerepresentations and are not necessarily drawn to scale.

Disclosed herein are solar cells having conductive contacts. In oneembodiment, a solar cell includes a substrate. A semiconductor region isdisposed in or above the substrate. A conductive contact is disposed onthe semiconductor region and can include a first conductive portioncomprising an alloy of a first metal and the substrate at an interfaceof the substrate and the semiconductor region.

Also disclosed herein are methods of fabricating solar cells havingconductive contacts. In one embodiment, a method of fabricating a solarcell involves forming a paste on a silicon substrate. The method canalso include depositing a metal (e.g., nickel) on the paste and firingthe nickel to form a first conductive portion that electrically couplesthe metal to the silicon substrate. The method can also include forminga conductive contact that includes the first conductive portion for thesolar cell.

This specification first describes an example solar cell that mayinclude the disclosed conductive contacts, followed by a more detailedexplanation of various embodiments of conductive contact structures. Foradditional context and to facilitate a better understand of thedisclosure, the specification then includes a description of varioussteps for fabricating a solar cell having conductive contacts. Last, thespecification includes a description of an example method for formingthe disclosed conductive contacts.

In a first exemplary solar cell, a seed layer is used to fabricatecontacts, such as back-side contacts, for a solar cell having emitterregions formed above a substrate of the solar cell. For example, FIG. 1Aillustrates a cross-sectional view of a portion of a solar cell havingconductive contacts formed on emitter regions formed above a substrate,in accordance with an embodiment of the present disclosure.

Referring to FIG. 1A, a portion of solar cell 100A includes patterneddielectric layer 224 disposed above a plurality of n-type dopedpolysilicon regions 220, a plurality of p-type doped polysilicon regions222, and on portions of substrate 200 exposed by trenches 216.Conductive contacts 228 are disposed in a plurality of contact openingsdisposed in dielectric layer 224 and are coupled to the plurality ofn-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222. The materials of, and methods offabricating, the patterned dielectric layer, the plurality of n-typedoped polysilicon regions 220, the plurality of p-type doped polysiliconregions 222, substrate 200, and trenches 216 may be as described belowin association with FIGS. 4A-4C.

Furthermore, the plurality of n-type doped polysilicon regions 220 andthe plurality of p-type doped polysilicon regions 222 can, in oneembodiment, provide emitter regions for solar cell 100A. Thus, in anembodiment, conductive contacts 228 are disposed on the emitter regions.In an embodiment, conductive contacts 228 are back contacts for aback-contact solar cell and are situated on a surface of the solar cellopposing a light receiving surface (direction provided as 201 in FIG.1A) of solar cell 100A. Furthermore, in one embodiment, the emitterregions are formed on a thin or tunnel dielectric layer 202, describedin greater detail in association with FIG. 4A.

Still referring to FIG. 1A, each of conductive contacts 228 can includeconductive layer 130, nickel (Ni) layer 132, and copper (Cu) layer 134.In one embodiment, conductive layer 130 can include a first conductiveportion that includes an alloy (e.g., nickel silicide) at an interfaceof the substrate and the semiconductor region. The nickel silicide canbe formed as a result of firing Ni layer 132. Examples of firing includeheating in a furnace or by laser annealing, among other examples.Conductive layer 130 can also include a paste (also referred to as aseed layer) in contact with the emitter regions of the solar cell 100A.In some embodiments, the paste may be a fired conductive paste layerwith the fired conductive paste layer being a second conductive portionof the conductive contact. In other embodiments, the paste may benon-conductive. The first and second conductive portions are illustratedin more detail in FIGS. 2A-2D and 3A-3D.

Turning now to FIGS. 2A-2D, a more detailed view of an exampleconductive contact is shown. As shown in FIG. 2A, paste 302 is depositedon substrate 300. As noted above, paste 302 may be a conductive ornon-conductive paste. Examples of conductive paste include aluminum,aluminum-silicon alloy, tin, conductive carbon, etc. As describedherein, a non-conductive paste may provide structural support onto whicha metal layer may be deposited. In various embodiments, the printedpaste can have a thickness of approximately 1-10 microns (may not beprinted in an equal or uniform distribution). Although metal paste 302is depicted as spherical particles for ease of illustration, theparticles may not necessarily be the same size or the same shape.

In one embodiment, depositing the paste may include printing (e.g.,screen printing, ink-jet printing, etc.) paste 302 on substrate 300. Thepaste may include a solvent for ease of delivery of the paste and mayalso include other elements, such as binders or frit.

To provide context, printed paste may be used as a low cost seed forsubsequent metal plating operations for solar cell metallization. Forexample, the paste can be printed in a pattern (e.g., a pre-determinedpattern consistent with the fingers or contact regions for the solarcell) such that the paste does not have to subsequently be masked andetched to form the pattern. Accordingly, printed seed paste may providea higher throughput lower cost technique than sputtered seed.

Printed conductive paste (e.g., metal paste, conductive carbon paste,etc.) can include conductive particles, which upon being fired, createelectrical contact to the silicon surface and create electrical contactamong the conductive particles. Melting the conductive particles cancreate a dense film with contact to the substrate but the melting canalso damage the actual wafer. If the conductive particles do not melt,it can be difficult to create a low contact resistance connection to thesubstrate and a low sheet resistance within the conductive particlefilm. These resistances can contribute to the solar cell seriesresistance and therefore limit the performance of the cell. As describedherein, the disclosed structures and techniques can improve theelectrical properties of the conductive contact of a solar cell.

As shown in the example of FIG. 2B, nickel 304 is deposited on paste302. In one embodiment, nickel may be plated according to an electrolessnickel plating technique. In other embodiments, other nickel depositiontechniques may be used to deposit the nickel.

FIG. 2C illustrates three first conductive portions 310 a-310 c thatresult from firing nickel layer 304. First conductive portions 310 a-310c can operate as a low contact resistance connection to the substrate.In an embodiment in which the nickel is plated above a silicon substrateand fired, first conductive portions 310 a-310 c include nickelsilicide. Note that in an embodiment in which the nickel is plated on aconductive paste, firing the nickel may also result in alloying of theconductive paste such that the first conductive portions include analloy of the conductive paste, nickel, and silicon substrate. Also notethat, as depicted in FIG. 2C, the nickel silicide may not be acontinuous layer across the whole contact. The discontinuous conductiveportions, though, may nevertheless be electrically coupled to oneanother such that a single conductive contact that includes theconductive portions can be formed.

FIG. 2D shows copper 306 deposited (e.g., plated) on nickel 304. Thefirst conductive portions may be referred to as the conductive layer, asin conductive layer 130, and collectively, the first conductiveportions, nickel, and copper may be referred to as a conductive contactof the solar cell.

In embodiments in which the paste is a conductive paste, firing thenickel can improve particle film conductivity by alloying the nickelwith the conductive particles to form alloys with low interfaceresistance between the particles and intercalated nickel films,resulting in a lowering of the sheet resistance for the particle/nickelfilm. In embodiments in which a non-conductive paste is used, a cheaperpaste that provides structural support for the plated nickel (or othermetal) can be used and a low firing temperature can also be used,thereby lowering risk of damage to the substrate.

FIGS. 3A-3D illustrate another example detailed view of a conductivecontact, according to one embodiment. As was the case in FIG. 2A, FIG.3A illustrates paste 302, which is a conductive paste in the example ofFIGS. 3A-3D, deposited on substrate 300. Different from FIG. 2B, FIG. 3Billustrates other conductive portions, second conductive portions 312a-312 e, which result from firing conductive paste 302. In addition tothe second conductive portions, FIG. 3B also illustrates variousparticles of the conductive paste at least partially melting together,which can provide greater continuity among the various conductiveparticles and from the conductive particles to the substrate.

After firing conductive paste 302, FIG. 3C illustrates nickel 304deposited on the fired metal paste. Similar to FIG. 2C, FIG. 3Dillustrates first conductive portions 310 a-h that result from firingnickel 304. Thus, the embodiment of a solar cell having conductivecontacts shown in FIG. 3D includes both first conductive portions 310and second conductive portions 312. Note that firing of the nickel mayalso result in alloying the conductive paste to the silicon substrate.As was the case with FIG. 2D, FIG. 3E illustrates copper 306 deposited(e.g., plated) on nickel 304.

Accordingly, in various embodiments (e.g., as shown in FIGS. 3A-3E), theconductive paste can be fired and the resulting fired paste can form thesecond conductive portions of conductive layer 130. In other embodiments(e.g., as shown in FIGS. 2A-2D), the paste may not be separately fired(not including the firing of the nickel deposited on the paste) and maynot have significant electrical connection from the paste to thesubstrate or between particles within the paste prior to nickeldeposition, instead serving as a support for the plated nickel. Asdescribed herein, the nickel can be fired to form the first conductiveportions of conductive layer 130 regardless of whether the metal pasteis also fired before the nickel is deposited. Also as described herein,because the nickel is plated to the paste, firing of the nickel may alsoresult in firing of the paste even though the paste may not be firedseparately before plating the nickel. For conductive pastes, theconductive paste may alloy with the silicon substrate and/or the nickelas a result of the nickel firing. The amount of alloying can varyaccording to the composition of the paste, the metal (e.g., nickel)plated to the paste, the substrate composition (e.g., silicon), and thefiring technique (e.g., temperature), among other variables.

Turning now to FIG. 1B, a cross-sectional view of a portion of anexample solar cell having conductive contacts formed on emitter regionsformed in a substrate is illustrated, according to one embodiment. Forexample, in this second exemplary cell, a seed layer is used tofabricate contacts, such as back-side contacts, for a solar cell havingemitter regions formed in a substrate of the solar cell.

As shown in FIG. 1B, a portion of solar cell 100B includes patterneddielectric layer 124 disposed above a plurality of n-type dopeddiffusion regions 120, a plurality of p-type doped diffusion regions122, and on portions of substrate 100, such as a bulk crystallinesilicon substrate. Conductive contacts 128 are disposed in a pluralityof contact openings disposed in dielectric layer 124 and are coupled tothe plurality of n-type doped diffusion regions 120 and to the pluralityof p-type doped diffusion regions 122. In an embodiment, diffusionregions 120 and 122 are formed by doping regions of a silicon substratewith n-type dopants and p-type dopants, respectively. Furthermore, theplurality of n-type doped diffusion regions 120 and the plurality ofp-type doped diffusion regions 122 can, in one embodiment, provideemitter regions for solar cell 100B. Thus, in an embodiment, conductivecontacts 128 are disposed on the emitter regions. In an embodiment,conductive contacts 128 are back contacts for a back-contact solar celland are situated on a surface of the solar cell opposing a lightreceiving surface, such as opposing a texturized light receiving surface101, as depicted in FIG. 1B.

In one embodiment, referring again to FIG. 1B, conductive contacts 128can include conductive layer 130, nickel (Ni) layer 132, and copper (Cu)layer 134. In one embodiment, as was the case in the example solar cellof FIG. 1A and as shown in FIGS. 2A-2D and 3A-3E, conductive layer 130can include one or more first conductive portions that include an alloy(e.g., nickel silicide) at an interface of the substrate and thesemiconductor region. The alloy can be formed as a result of firing Nilayer 132. Conductive layer 130 can also include a paste in contact withthe emitter regions of the solar cell 100A. In some embodiments, thepaste may be a fired metal paste (e.g., fired before firing the nickel)with the fired paste resulting in second conductive portions of theconductive contact. In other embodiments, the paste may serve asstructural support for the deposited nickel and not provide additionalelectrical connectivity for the conductive contact.

Although certain materials are described specifically above withreference to FIGS. 1A, 1B, 2A-2D, and 3A-3E, some materials may bereadily substituted with others with other such embodiments remainingwithin the spirit and scope of embodiments of the present disclosure.For example, in an embodiment, a different material substrate, such as agroup III-V material substrate, can be used instead of a siliconsubstrate. In such an embodiment, instead of nickel silicide beingformed from firing the nickel layer, another conductive layer can beformed from fired nickel and the different material substrate. Asanother example, in one embodiment, silver (Ag) particles, carbon (C),tin, or the like can be used in a seed paste in addition to or insteadof Al particles. In another embodiment, plated or like-deposited cobalt(Co) or tungsten (W) can be used instead of or in addition to the Nilayer described above. As is the case with the Ni layer, the Co or W maybe fired resulting in the first conductive portions.

Note that, in various embodiments, the formed contacts need not beformed directly on a bulk substrate, as was described in FIG. 1B. Forexample, in one embodiment, conductive contacts such as those describedabove are formed on semiconducting regions formed above (e.g., on a backside of) as bulk substrate, as was described for FIG. 1A. As an exampleand to provide additional context to the fabrication of a solar cell,FIGS. 4A-4C illustrate cross-sectional views of various processingoperations in one example embodiment of fabricating solar cells havingconductive contacts.

Referring to FIG. 4A, fabricating a back-contact solar cell can includeforming thin dielectric layer 202 on substrate 200. In one embodiment,thin dielectric layer 202 is composed of silicon dioxide and has athickness approximately in the range of 5-50 Angstroms. In oneembodiment, thin dielectric layer 202 performs as a tunnel oxide layer.In an embodiment, substrate 200 is a bulk monocrystalline siliconsubstrate, such as an n-type doped monocrystalline silicon substrate.However, in an alternative embodiment, substrate 200 includes apolycrystalline silicon layer disposed on a global solar cell substrate.

Referring again to FIG. 4A, trenches 216 can be formed between n-typedoped polysilicon regions 220 and p-type doped polysilicon regions 222.Portions of trenches 216 can be texturized to have textured features218, as is also depicted in FIG. 4A. Dielectric layer 224 can be formedabove the plurality of n-type doped polysilicon regions 220, theplurality of p-type doped polysilicon regions 222, and the portions ofsubstrate 200 exposed by trenches 216. In one embodiment, a lowersurface of dielectric layer 224 can be formed conformal with theplurality of n-type doped polysilicon regions 220, the plurality ofp-type doped polysilicon regions 222, and the exposed portions ofsubstrate 200, while an upper surface of dielectric layer 224 issubstantially flat, as depicted in FIG. 4A. In a specific embodiment,the dielectric layer 224 is an anti-reflective coating (ARC) layer.

Referring to FIG. 4B, a plurality of contact openings 226 is formed indielectric layer 224. The plurality of contact openings 226 providesexposure to the plurality of n-type doped polysilicon regions 220 and tothe plurality of p-type doped polysilicon regions 222. In oneembodiment, the plurality of contact openings 226 is formed by laserablation. In one embodiment, the contact openings 226 to the n-typedoped polysilicon regions 220 have substantially the same height as thecontact openings to the p-type doped polysilicon regions 222, asdepicted in FIG. 4B.

Referring to FIG. 4C, the method of forming contacts for theback-contact solar cell further includes forming conductive contacts 228in the plurality of contact openings 226 and coupled to the plurality ofn-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222. Thus, in an embodiment, conductivecontacts 228 are formed on or above a surface of a bulk N-type siliconsubstrate 200 opposing a light receiving surface 201 of the bulk N-typesilicon substrate 200. In a specific embodiment, the conductive contactsare formed on regions (222/220) above the surface of the substrate 200,as depicted in FIG. 4C. The conductive contacts can be the conductivecontacts described herein at FIGS. 1A, 1B, 2A-2D, 3A-3E and 5.

Turning now to FIG. 5, a flow chart illustrating a method for forming aconductive contact is shown, according to some embodiments. In variousembodiments, the method of FIG. 5 may include additional (or fewer)blocks than illustrated. For example, in some embodiments, firing thepaste before the nickel is deposited as shown at 504 may not beperformed.

As shown at 502, a paste can be formed. Forming the paste can includeprinting the paste in a pattern that is suitable for connectivity withrespective p and n regions of a solar cell (e.g., in a pre-determinedfinger pattern). In one embodiment, the paste is formed from a mixturethat includes conductive particles (e.g., metal particles), a liquidbinder, and an inert filler material. In other embodiments as describedherein, the composition of the paste may be different (e.g.,non-conductive particles).

At 504, in one embodiment, the paste can be fired (e.g., heated, laserannealed, etc.) to form a conductive portion of the conductive contactthat electrically couples the paste to the silicon substrate. Forexample, firing can be performed by heating according to a temperaturebased on the composition of the paste. For instance, for a metal pastethat includes Al/Si particles, the firing temperature may beapproximately 550 degrees Celsius. As described herein, the result offiring the paste can be the formation of a conductive portion of theconductive contact. Note that the conductive portion can be irregularand not a uniform layer, as illustrated by the five square regions inFIGS. 3B-3E.

As illustrated at 506, a metal layer, such as a nickel layer, can bedeposited on the paste. For example, in one embodiment, nickel can bedeposited on the metal paste layer according to an electroless nickelplating technique.

As shown at 508, the nickel may be fired (e.g., heated, laser annealed,etc.) to form a conductive portion of the conductive contact thatelectrically couples the nickel to the substrate. Silicides can beformed from nickel with low specific contact resistance values over abroad range of temperatures, from as low as about 300 degrees Celsius.An example illustration of such a conductive portion can be seen inFIGS. 2C and 2D. In embodiments in which the metal paste layer and thenickel layer are fired at blocks 504 and 508, respective conductiveportions may be formed as a result of each firing. As described herein,such conductive portions are shown in FIGS. 3D and 3E. Note that firingthe nickel 508 can also fire the paste on which the nickel is platedeffectively double firing the paste if the paste was fired at block 504.The firing temperature, however, may not be high enough to significantlyalloy the paste (if a conductive paste) to the nickel or siliconsubstrate.

At 510, another metal layer can be plated to the metal layer. Forexample, in one embodiment, copper can be plated to nickel that wasplated to the paste at 506.

The disclosed structures and techniques can improve the electricalproperties of conductive contacts. For example, by firing a wafer afterdepositing nickel according to the disclosed structures and techniques,contact resistance can be decreased by the silicidation reaction betweennickel and silicon at the nickel/silicon interface. Additionally, thecombined paste/nickel sheet resistance can also be decreased by alloyingthe nickel and metal particles thereby creating a less porous,continuous metal film. Further, in forming the nickel silicide, nickelis the diffusing species so pitting or spiking of the silicon substratecan be eliminated, minimized, or reduced. Additionally, nickel can alsofunction as a barrier layer to copper diffusion.

Moreover, in embodiments in which the paste is not first fired, concernsover achieving adequate contact resistance and sheet resistance valuesfrom a paste without damaging the substrate can be removed becausesufficient contact and sheet resistance can be achieved through firingthe nickel at a lower temperature than the paste would be fired. Thus, acheaper and/or non-conductive paste can be used as a support instead ofrelying on the paste for conductivity in the contact.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. A method of fabricating a solar cell, the methodcomprising: forming a paste on a silicon substrate; depositing nickel onthe paste; firing the nickel to form a plurality of discontinuousconductive portions that electrically couples the nickel to the siliconsubstrate and alloys the paste, nickel and silicon substrate at aninterface of the substrate; and forming a conductive contact thatincludes one of the plurality of conductive portions for the solar cell.2. The method of claim 1, further comprising: before said depositing thenickel, firing the paste to form a second conductive portion thatelectrically couples the paste to the silicon substrate, wherein theconductive contact also includes the second conductive portion.
 3. Themethod of claim 1, wherein said forming the conductive contact includeselectroplating copper on the nickel.
 4. The method of claim 1, whereinsaid forming the paste includes printing a conductive paste.
 5. Themethod of claim 1, wherein said forming the paste includes printingparticles that include aluminum.
 6. The method of claim 1, wherein saidfiring the nickel to form the first conductive portion also electricallycouples the paste to the silicon substrate.
 7. A method of fabricating asolar cell, comprising: depositing a paste on a semiconductor regiondisposed in or above a silicon substrate; plating a first metal on thepaste; forming a conductive alloy from the first metal, paste and thesilicon substrate by firing; plating a second metal on the first metal,wherein the first and second metals and the conductive alloy form aconductive contact for the semiconductor region of the solar cell; wherethe firing step forms a plurality of discontinuous conductive alloyportions.
 8. The method of claim 7, further comprising: before saidplating the first metal on the paste, forming another conductive alloyfrom the paste and the silicon substrate, wherein the conductive contactis also formed from said another conductive alloy.
 9. The method ofclaim 7, wherein said depositing the paste includes printing particlesof a conductive paste in a pre-determined finger pattern.
 10. The methodof claim 7, wherein said plating the first metal includes electrolesslyplating nickel on the paste.
 11. The method of claim 7, wherein saidforming the conductive alloy includes heating nickel to form nickelsilicide.